//单例模式
//    亦称： 单件模式、 Singleton
//    意图
//
//单例模式是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。

#include <iostream>
#include <thread>

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{

  /**
   * The Singleton's constructor should always be private to prevent direct
   * construction calls with the `new` operator.
   */

protected:
  Singleton(const std::string value): value_(value)
  {
  }

  static Singleton* singleton_;

  std::string value_;

public:

  /**
   * Singletons should not be cloneable.
   */
  Singleton(Singleton &other) = delete;
  /**
   * Singletons should not be assignable.
   */
  void operator=(const Singleton &) = delete;
  /**
   * This is the static method that controls the access to the singleton
   * instance. On the first run, it creates a singleton object and places it
   * into the static field. On subsequent runs, it returns the client existing
   * object stored in the static field.
   */

  static Singleton *GetInstance(const std::string& value);
  /**
   * Finally, any singleton should define some business logic, which can be
   * executed on its instance.
   */
  void SomeBusinessLogic()
  {
    // ...
  }

  std::string value() const{
    return value_;
  }
};

Singleton* Singleton::singleton_= nullptr;;

/**
 * Static methods should be defined outside the class.
 */
Singleton *Singleton::GetInstance(const std::string& value)
{
  /**
   * This is a safer way to create an instance. instance = new Singleton is
   * dangeruous in case two instance threads wants to access at the same time
   */
  if(singleton_==nullptr){
    singleton_ = new Singleton(value);
  }
  return singleton_;
}

void ThreadFoo(){
  // Following code emulates slow initialization.
  std::this_thread::sleep_for(std::chrono::milliseconds(1000));
  Singleton* singleton = Singleton::GetInstance("FOO");
  std::cout << singleton->value() << "\n";
}

void ThreadBar(){
  // Following code emulates slow initialization.
  std::this_thread::sleep_for(std::chrono::milliseconds(1000));
  Singleton* singleton = Singleton::GetInstance("BAR");
  std::cout << singleton->value() << "\n";
}


int main()
{
  std::cout <<"If you see the same value, then singleton was reused (yay!\n" <<
            "If you see different values, then 2 singletons were created (booo!!)\n\n" <<
            "RESULT:\n";
  std::thread t1(ThreadFoo);
  std::thread t2(ThreadBar);
  t1.join();
  t2.join();

  return 0;
}